The low loss and dispersion of single-mode fiberguides make them attractive for optical communications systems having long repeater spacing and large bandwidth. In such systems, the maximum data transmission rate is limited by chromatic dispersion due to material and waveguide effects.
It is well known that in pure silica, as well as in doped silica such as is presently used in the manufacture of optical fiberguide, the slope of the material dispersion as a function of wavelength is positive for the wavelength range of current interest for optical communications systems, i.e., from about 0.8 .mu.m to about 1.6 .mu.m. Furthermore, the material dispersion is zero at about 1.3 .mu.m. In contrast, the waveguide dispersion typically has a small negative slope for the same wavelength range in single-mode fiberguide. Thus, total chromatic dispersion, which, to a first approximation, is the algebraic sum of material dispersion and waveguide dispersion, also has a zero, albeit typically at a wavelength different from that of the material dispersion zero. See, for instance W. A. Gambling et al., Electronics Letters, Vol. (15), pp. 474-476, 1979. It is also well recognized that the loss spectrum of silica-based fiberguide typically has a relative minimum at about 1.3 .mu.m, and an absolute minimum (for the wavelength regime of interest here) at about 1.55 .mu.m. Calculations have shown that it is possible to design single-mode step-index fiberguide having zero chromatic dispersion at the wavelength of minimum loss. C. P. Chang, Electronics Letters, Vol. 15(23), pp. 765-767, 1979.
Although a single mode fiberguide in which the minimum of chromatic dispersion occurs at the wavelength of minimum loss near 1.55 .mu.m has high bandwidth at that wavelength, such a fiberguide typically has to have a very small core, typically of less than about 5 .mu.m diameter, and difficult splicing problems result. Furthermore, in a communication system using such fiberguide, even a very slight deviation of signal carrier wavelength from the wavelength of minimum dispersion results in substantial degradation of the system's bandwidth. For instance, a carrier deviation of .+-.0.05 .mu.m from the wavelength of minimum dispersion results in a reduction of the maximum data transmission rate by almost two orders of magnitude. It is clear from this discussion that standard step-index single-mode fiberguide designed for operation at 1.3 .mu.m is not suitable for operation near 1.55 .mu.m, and vice versa, and similarly, that such fiberguide typically would not permit wavelength-division multiplexing.
It has recently been recognized that double-clad single-mode fiberguide, also referred to as W-profile fiberguide, has potentially substantial advantages over conventional single-clad guide. For instance, it has been pointed out that W-profile fiberguide is capable of yielding single-mode operation with a larger core than is possible in conventional single-mode step-index guide. S. Kawakami and S. Nishida, IEEE Journal of Quantum Electronics, QE-10(12), pp. 879-887, 1974. More recently, it has also been realized that single-mode W-profile fiberguide can be designed to have two zeroes of the total chromatic dispersion in the relevant wavelength regime with a finite but small dispersion for the wavelength region between the two zeroes. K. Okamoto et al., Electronics Letters, Vol. 15(22), pp. 729-731, 1979.
However, these results were obtained for fiberguides having rather undesirable parameter values. In particular, the refractive index differences between the relevant fiber regions are guite large, of the order of 1 percent, and the core diameter is quite small, about 7 .mu.m. Large index differences imply heavy doping, which typically results in substantial Rayleigh scattering losses, and small cores typically result in substantial splicing losses. Both of these loss mechanisms of course tend to reduce the possible repeater spacings.
Very recently, T. Miya et al., IEEE Journal of Quantum Electronics, Vol. QE-17(6), pp. 858-861 (1981), reported double-clad W-profile fiberguide having smaller index differences and larger core size than the fiberguide discussed by Okamoto et al. (op cit.). In particular, Miya et al. report on single-mode W-profile fiberguides with an index difference between core and outer cladding of, e.g., 0.52 percent, an index difference between outer and inner cladding of -0.31 percent, a core diameter of about 7-8 .mu.m, and a thickness of the inner cladding approximately equal to the core radius. In these fibers, the core was up-doped with germania, and the inner cladding was down-doped with fluorine. Because of the multiplicity of dopants used, fabrication of such fibers is typically relatively complex. Furthermore, the relatively heavily doped core region results in relatively high Rayleigh scattering, and the doping shifts the wavelength of zero material dispersion of the core to a wavelength above 1.3 .mu.m, typically causing the short-wavelength minimum of the chromatic dispersion also to occur at a wavelength above 1.3 .mu. m. Because of these and other reasons, it appears desirable to find ranges of parameters that will yield fiberguide having larger cores and reduced Rayleigh scattering, and that give greater freedom in tailoring of the chromatic dispersion spectrum.